There is a need for an electro-optical device that in one or more light states is transparent to visible light and in other light states attenuates light, and in particular for a device that transmits a high level of light in an extreme light state. Applications for such a device include use as a light attenuator in a smart glass, use as a see-through display, or use as a sunlight-readable, reflective display.
It is desirable to control light transmittance in windows, and to provide functions including: see-through (i.e. transparent), privacy (opaque), variable tinting or dimming, or black-out (light transmittance of a few percent or less). In the prior art, the available active window solutions (i.e. selectable or switchable light control) have limited functionality and inherent technological obstacles. In display devices it is desirable to extend functionality into new areas such as providing see-through (i.e. transparent displays), or providing large-format, sunlight-readable, reflective displays for outdoor applications.
Conventional electrophoretic displays move oppositely charged particle sets in a suspending fluid in the direction of an electrical field. This is normally an orthogonal field and a transparent state is not possible. Inherent in the addressing of these displays is the need to flash or blank a screen each time it is updated to achieve repeatable, grey-scale, light states. The white state in conventional electrophoretic displays can appear grey instead of white because they rely on white charged particles to reflect light, and typically the reflected light is about 50% of the incident light on a particle. By contrast, a diffuse white reflector layer revealed by a highly transparent state could provide up to 100% reflectance within the aperture area revealed.
One proposal to create a transparent state in an electrophoretic device is to finely pattern one or both electrodes. This allows the charged particles to be moved laterally as well as vertically and to collect on patterned electrodes (corresponding to a subset of a display area) that have a pitch of about 200 to 300 microns. The area between patterned electrodes is then transparent and provides visual access. In some examples of display devices employing patterned electrodes just one substrate has electrodes and particles move laterally between neighbouring electrodes with one electrode group accounting for about 70% of the area. But while the fine patterning of electrodes is normal for display devices having a matrix of pixels, it adds undesirable complexity. In a light attenuator for smart glass applications fine patterning of electrodes is prohibitively expensive.
Examples of electrophoretic devices that may have an inherent transparent light state capable of transmitting light and providing visual access include electrophoretic devices that use a dielectrophoretic effect to collect charged particles at a side wall of a capsule in one light state; or electrophoretic devices that form microstructures in place (e.g., using one of the following processes: embossing with a directly-formed tool, photolithography, extruding, or laser micromachining) to collect charged particles in one light state; or electrophoretic devices that use the dispersal (i.e. in a suspending fluid volume) of 10 nm to 50 nm scale, charged nanoparticles to transmit light and provide visual access in one light state; or hybrid electrophoretic devices (called electrokinetic by their inventors at Hewlett Packard) that use photolithographically created micro-pits to collect charged particles in one light state. The feasibility of these prior art electrophoretic technologies for a light attenuator in a smart glass application is questionable due to the efficacy of their transparent light state or their complexity, and for some of these technologies in display devices their complexity limits them to small-area applications. For example, one proponent of replicated microstructures proposes making embossing moulds on silicon. This would seem to limit such devices to small areas and discrete or batch manufacturing, and such tooling would not seem suited to large-format, roll-to-roll manufacturing.
In conclusion, there is a need for an electrophoretic device that has an inherent transparent light state and provides variable control of light attenuation. Its construction and method of manufacture needs to be compatible with roll-to-roll manufacture, and a web width of about 1M for smart glass applications. Its operation should provide seamless variable light control in applications ranging from smart glass to active-matrix displays and avoid the need for flashing or blanking.